Certain types of equipment, and manufacturing processes, analytical chemistry techniques and other similar operations can be adversely affected in the presence of air containing even normal levels of dust and other airborne particulate matter. Accordingly, dust-sensitive equipment and processes are generally maintained or conducted in a controlled environment which is provided with filtered air from which all or most particulate material above a predetermined minimum size has been removed. Such controlled environments wherein reduced particulate levels are maintained are commonly referred to as "Clean-Rooms". Filtered, conditioned air is generally supplied to the Clean-Room at a rate which is sufficient to maintain a substantially continuous positive pressure (i.e., above ambient pressure) in the Clean-Room so that any leakage of air between the Clean-Room and its surroundings occurs outwardly from the Clean-Room. To minimize the cost of supplying filtered, conditioned (e.g., heated, cooled, humidified, dehumidified, etc.) air to a Clean-Room at a pressure and rate which is sufficient to maintain a positive pressure in the Clean-Room and to prevent infiltration of ambient particulate-laden air into the Clean-Room, it is highly desirable to eliminate nonessential openings which allow filtered, conditioned air to escape from the Clean-Room or which could allow particulate-laden air to enter. It is also desirable to maintain good seals around any essential or desired openings such as doors, windows, and passageways for electrical conduits, plumbing, etc.
Drop-ceilings are often used in Clean-Rooms for aesthetic and practical reasons, such as to conceal conduit, piping, ductwork, etc., to eliminate the need to maintain air-tight seals at openings through which conduit, piping, ductwork and the like would otherwise enter the Clean-Room and to reduce the volume of air which must be pumped and filtered to maintain the desired particle-free environment. In order to minimize leakage from the Clean-Room and provide improved isolation of the Clean-Room from particulate-laden air in the surrounding environment, specially designed sealed lighting units are used for Clean-Room drop-ceilings. Known types of such sealed lighting units are much more expensive than ordinary drop-ceiling lighting units because the sealed lighting units are designed to have a completely sealed outer housing. This places the light-emitting means (e.g., fluorescent bulbs) inside the filtered atmosphere of the Clean-Room, and allows for replacement of the light-emitting means without exposing the Clean-Room to contamination from the outside, but this is an incidental benefit and the main goal is to merely maintain a continuous wall between the inside of the Clean-Room and the outside.
Housings for conventional Clean-Room drop-ceiling lighting units are generally formed from sheet metal by conventional cutting and bending operations, as are ordinary or general purpose drop-ceiling lighting units. However, unlike ordinary lighting units, the adjacent sidewall edges of housings for sealed Clean-Room drop-ceiling lighting units are seam-welded at all points to provide fluid-tight corners, which are subsequently ground smooth for aesthetic and safety reasons. These additional operations add a great deal to the production costs of conventional Clean-Room drop-ceiling lighting units as compared with ordinary lighting units wherein the corners are not fluid-tight. The additional expense associated with the seam-welded corners on the housings of conventional Clean-Room drop-ceiling lighting units has heretofore been generally regarded as necessary and unavoidable to reduce air leakage from the Clean-Room and to ensure that particulate contaminants present in the space above the drop-ceiling do not enter into the Clean-Room through the lighting units.